The present invention relates to a process for filtering a three-phase reaction mixture comprising a liquid phase, a non-dissolved solid catalytic phase and a gaseous phase.
In processes using a compound comprising at least one nitrile function and a non-dissolved catalyst of Raney nickel or cobalt type, it has been observed that in the absence of gas, and more particularly in the absence of hydrogen, the nitrile functions of the medium have a harmful influence on the activity of the catalyst.
Thus, in particular in the processes for the hydrogenation of nitriles into amines, and even more particularly in processes for the partial hydrogenation of a dinitrile into aminonitrile, the Applicant has demonstrated that the catalyst of Raney nickel or cobalt type has a tendency to become deactivated in the presence of nitrile functions, when the medium no longer or only sparingly contains hydrogen.
Moreover, it turns out that metal catalysts deposited on a support, which are also used, in particular in hydrogenation processes, separate out poorly by settling and are difficult to filter.
The present invention proposes a solution to these problems, which consists in tangentially filtering, on a membrane filter, at least some of a three-phase reaction mixture comprising a liquid phase in which nitrile functions are found, a gaseous phase comprising hydrogen and a catalytic solid phase comprising Raney nickel and/or cobalt or a supported metal catalyst, and in recycling the catalyst while at the same time recovering at least some of the filtrate containing the reaction products.
The tangential circulation, relative to the membrane and at high speed, of the mixture to be filtered minimizes the amount of solid retained on the filter. It is thus possible to minimize the risks of deactivation of the catalyst. It allows continuous filtration while at the same time keeping the catalyst in its reaction medium.
Generally, the circulation speed of the reaction mixture through the membrane is between 0.5 meter/second and 20 meters/second and preferably between 1 meter/second and 10 meters/second.
The membrane filters used for the tangential filtration generally consist of a flat or tubular support and an inorganic or organic membrane, often referred to as the active layer, which is a few micrometers in thickness.
For the process of the invention, the membranes used will preferably be inorganic membranes which generally behave better chemically and thermally with respect to the reaction mixture used.
The support and the active layer can be made of the same material or of different materials. The active layer of the filter can be made, for example, of xcex1-alumina, zirconium oxide, titanium dioxide or graphite fibres. The support can also be made of alumina, graphite, zirconium oxide or titanium dioxide.
The membrane is also characterized by its mean pore diameter. Generally, this mean pore diameter ranges between 1 nanometer and one micrometer.
For reasons of lifetime and regenerability of the membrane, it is preferred in the present process to use ultrafiltration membranes which have a mean pore diameter of from 10 nanometer to 100 nanometer and a cutoff threshold (which is defined by the molecular mass of the compounds stopped by the membrane and is expressed in grams/mole or daltons) of greater than or equal to 5 kilodaltons (kD) and preferably greater than or equal to 100 kilodaltons.
The liquid phase of the reaction mixture to be filtered essentially comprises at least one compound containing nitrile functions, such as the unconverted starting dinitrile or nitrile, the aminonitrile and/or the amine and/or the diamine formed, and an optional solvent which can be, in particular, water and/or an amide and/or an alcohol and/or an amine and/or ammonia. The alcohols usually used are alkanols such as methanol, ethanol, 1-propanol, 2-propanol and 1-butanol, diols such as ethylene glycol and propylene glycol, polyols or mixtures of the said alcohols. When the solvent is an amide, use may be made in particular of dimethylformamide and dimethylacetamide. Among the amines which can be used as solvent, mention may be made, for example, of the amine, the diamine or the aminonitrile corresponding to the nitrile or the dinitrile which is hydrogenated. The liquid phase also generally comprises a strong inorganic base derived from an alkali metal or alkaline-earth metal.
When water is present with another solvent, the said solvent represents, on a weight basis, from two to four times the weight of water.
The solid catalytic phase generally consists of a catalyst based on Raney nickel and/or Raney cobalt, optionally, but preferably, comprising a doping element chosen from the elements from Groups IVb, VIb, VIIb and VIII of the Periodic Table of the Elements as published in the Handbook of Chemistry and Physics, 51st edition (1970-1971).
The catalyst based on Raney nickel and/or Raney cobalt used in the process can thus comprise, besides nickel or cobalt and the residual amounts of the metal eliminated from the original alloy during the preparation of the catalyst, i.e., generally, aluminium, one or more other doping elements, such as, for example, chromium, titanium, molybdenum, tungsten, iron or zinc.
Among these doping elements, chromium and/or iron and/or titanium are considered as the most advantageous. These doping elements usually represent, on a weight basis relative to the weight of nickel, from 0% to 15% and preferably from 0% to 10%.
The catalytic phase can also consist of a metal, which is generally a metal from Group VIII of the Periodic Table of the Elements, such as ruthenium, rhodium, nickel or cobalt, deposited on a support which is generally a metal oxide, such as aluminas, silicas, aluminosilicates, titanium dioxide, zirconium oxide or magnesium oxide.
In the supported metal catalysts, the metal generally represents from 0.1 to 80% of the weight of the support, and preferably from 0.5 to 50%.
The solid phase generally represents from 1% to 50% by weight relative to the weight of the liquid phase, without these values being critical.
The gaseous phase consists essentially of hydrogen.
The tangential filtration can be carried out at a temperature which is advantageously that at which the hydrogenation reaction is carried out. This temperature is usually less than or equal to 150xc2x0 C., preferably less than or equal to 120xc2x0 C. and even more preferably less than or equal to 100xc2x0 C.
In concrete terms, this temperature is usually between room temperature (about 20xc2x0 C.) and 100xc2x0 C. The process can be performed, without any technical difficulties, at a temperature below 20xc2x0 C., but there is no advantage in doing so on account of the lower production efficiency of the hydrogenation reaction.
The pressure difference between the filter inlet and outlet, which is required for the filtration, can be provided in part by the pressure at which the hydrogenation reaction is carried out. However, it is necessary to create a pressure greater than atmospheric pressure. Generally, this pressure is between 1 bar (0.10 MPa) and 20 bar (1 MPa) and preferably between 2 bar (0.5 MPa) and 10 bar (5 MPa). In practice, the pressure is created by a pump which feeds the membrane filter from the reactor.
The flow feeding the membrane filter obviously depends on the amount of reaction mixture present in the reactor, as well as the capacity of the filter. It is also determined according to the reaction progress, such that the liquid filtrate or permeate, which will be at least partially recovered and treated elsewhere, contains sufficient amounts of the intended product(s) of the hydrogenation reaction.
The catalyst filtered in the presence of hydrogen remains active and is recycled into the hydrogenation reaction.